A bulb socket known to the prior art is shown in FIG. 16. Bulb 1 includes flat base 2 and lead wires 3, provided at each of the opposite side surfaces of base 2. Bulb socket 4 includes cylindrical bulb receptacle 5 having an opening to accommodate base 2 therein. A pair of fittings 6 are disposed along the inner surface of bulb receptacle 5.
Guide plates 6a are inserted along groove 5a formed in receptacle 5 to properly position fitting 6. A rear part of guide plate 6a with respect to its insertion direction is cut and bent outward, thereby forming engaging portion 6a1. An angular portion at the leading end of engaging portion 6a1 digs into the inner wall of receptacle 5, thereby preventing fitting 6 from easily coming out of receptacle 5. Each fitting 6 is also formed with retaining portion 6b having an arm-like configuration which extends in its insertion direction. Retaining portion 6b has at its leading end contact portion 6b1 which is triangularly curved to project inward of receptacle 5 and is engageable with corresponding projection 2a formed on the base 2.
In this bulb socket, when base 2 of bulb 1 is inserted into bulb receptacle 5, projections 2a come into contact with retaining portion 6b of fittings 6, thereby bending retaining portion 6b outward. When base 2 is inserted deeper into bulb receptacle 5, retaining portion 6b moves over corresponding projections 2a to lock bulb 1 in place. In this position, base 2 is tightly retained by fittings 6 between portion 6b and projections 2a, thereby to prevent base 2 from slipping out of bulb receptacle 5.
When wedge-base bulb 1 is withdrawn, projections 2a press contact portions 6b1 as they move upward, thereby forcing contact portions 6b1 outward. As a result, contact portions 6b1 move over projections 2a. Although contact portions 6b1 are pulled upward by the projections 2a at this stage, the angular portions at the leading ends of engaging portions 6a1 of guide plates 6a dig into the inner walls of receptacle 5 to restrict the upward movement of contact portions 6b1.
However, if the projections 2a of the base 2 are large, it becomes difficult for them to move over contact portions 6b1. Thus, the force used to pull bulb 1 upward acts to pull the contact portions 6b1 upward. Since the size tolerance of the bulb 1 is large, there are cases where fittings 6 cannot be retained in receptacle 5 only by engaging portions 6a1. As a result, fittings 6 are pulled out of receptacle 5 when bulb 1 is withdrawn.
The foregoing conventional bulb socket retains a part of the electric bulb by bending the fittings. However, electric bulbs are subject to variations in dimensions. Even if the bulb socket is fabricated to conform to the standard size, the bulb may vary within manufacturing tolerances. When the bulb, especially its socket or the projections thereon, is oversized, the fittings may be bent beyond the elastic limit and be permanently distorted. When the electric bulb is undersized, the forces exerted thereon by the fitting may be too weak so that the bulb may fall out of or be shaken loose from the socket. This tendency is of particular concern in relation to wedge-base bulbs or the like which are made entirely of glass because the manufacturing tolerances thereof are larger.